Reland "Refactors BitrateProber with unit types and absolute probe time."

This is a reland of 739a5b3692880cb6b41ae620fb9e755c39b044b1

Patchset 1 is the original CL, patchset 3 includes a fix

Original change's description:
> Refactors BitrateProber with unit types and absolute probe time.
>
> Using unit types improves readability and some conversion in PacedSender
> can be removed.
>
> TimeUntilNextProbe() is replaced by NextProbeTime(), so returning an
> absolute time rather than a delta. This fits better with the upcoming
> TaskQueue based pacer, and is also what is already stored internally
> in BitrateProber.
>
> Bug: webrtc:10809
> Change-Id: I5a4e289d2b53e99d3c0a2f4b36a966dba759d5cf
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158743
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29670}

Bug: webrtc:10809
Change-Id: I033193c78474fdd82c109fdab0a8f09a05f7b30e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158841
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29688}

modules/pacing/pacing_controller.h

@@ -118,10 +118,11 @@ class PacingController {
   // effect.
   void SetProbingEnabled(bool enabled);
 
-  // Time until next probe should be sent. If this value is set, it should be
+  // Time at which next probe should be sent. If this value is set, it should be
   // respected - i.e. don't call ProcessPackets() before this specified time as
   // that can have unintended side effects.
-  absl::optional<TimeDelta> TimeUntilNextProbe();
+  // If no scheduled probe, Timestamp::PlusInifinity() is returned.
+  Timestamp NextProbeTime();
 
   // Time since ProcessPackets() was last executed.
   TimeDelta TimeElapsedSinceLastProcess() const;